1. Field of the Invention
The invention relates to a light source device using a high pressure mercury discharge lamp which can be used as a light source for a projector.
2. Description of Related Art
In a light source device for an optical device, such as a liquid crystal projector, a DLP(trademark) projector (Texas Instruments) or the like, a discharge lamp with high radiance (HID lamp) is used. Recently there has been a need for a greater amount of mercury to be added to the discharge lamp than was conventional in the past in order to make the optical device more radiant. In one such discharge lamp, it is necessary to use a starter which operates at a high voltage when starting and subjects the discharge space light source to insulation breakdown which results in a discharge.
FIG. 12 illustrates the arrangement of such a conventional discharge lamp light source device. In a light source device for an optical device, conventionally a starter (Ui) is used by which a high voltage is applied between the electrodes (E1, E2) of the two poles. In this system, the secondary winding (Si) of the high voltage transformer (Ti) of the starter is series connected to a lamp (Li). After a discharge begins, operation of the starter (Ui) is no longer necessary. The discharge current supplied to the lamp (Li) must nevertheless flow via the secondary winding (Si) of the high voltage transformer having a large number of windings. To suppress the formation of heat loss in the winding (Si), it is necessary to make the winding wire diameter large, resulting in the disadvantage that an increase in the size and weight of the starter (Ui) is inevitable.
As a means for eliminating this disadvantage, an outside trigger method, which is often used for the triggering of a blinking lamp, can be used. In this method, in addition to the first electrode and the second electrode of the two poles for the main discharge, i.e. the arc discharge after starting, there is provided an auxiliary electrode between which a high voltage is applied to the first or the second electrode. Plasma is produced by a dielectric barrier discharge in the discharge space between the first electrode and the second electrode by means of a voltage applied beforehand (no-load voltage) then a main discharge is started, such that these plasmas act as the triggering substance.
After starting the lamp discharge, no lamp discharge current flows in the primary winding and the secondary winding of the high voltage transformer of the starter in this arrangement. Consequently, there is no heat loss in the primary and secondary windings of the high voltage transformer of the starter. Therefore, both an increase in the size and weight of the starter can be prevented.
In a discharge lamp containing a large amount of mercury, however, the pressure in the discharge space is low due to condensation of the mercury when the lamp is at room temperature. In this situation, starting can take place rather easily. However, in the situation of a hot lamp, that is immediately after being turned off, the pressure in the discharge space is high because the mercury has been vaporized. The disadvantage here is that restart (hot restart) is difficult.
The disadvantage associated with a difficult restart under hot restart conditions is serious for an optical device, such as a projector or the like, with respect to the adverse effect on the ease of re-use by a user of the device. The disadvantage of a difficult restart has become more and more serious in recent years for the outside trigger method due to increase in the amount of mercury contained in the device.
On the other hand, conventionally the discharge lamp (Li) and a feed device (Ni) were connected to one another by supply lines (K1, K2) with the starter (Ui) for starting the discharge lamp (Li) being located on the feed device (Ni) side in order that the starter (Ui) can generate a high voltage. In the case in which the starter (Ui) generates a pulsed high voltage, the feed lines (K1, K2) are quickly placed under a high voltage, resulting in the disadvantage that strong noise was emitted.
Furthermore, dulling distortion of the pulsed high voltage is caused by the electrostatic capacity which is formed between the feed lines (K1, K2) and the conductor, and by the inductance of the feed lines (K1, K2). As a result, the voltage increase between the lamp electrodes (E1, E2) is reduced. In order to obtain the pulsed voltage necessary for starting the discharge lamp, a greater energy than necessary must be emitted by the starter (Ui) onto the feed lines (K1, K2). In addition, the pulse width is broadened by dulling distortion of the pulsed high voltage. This increases the possibility of an insulation breakdown in an unintended area, such as the insulation coating of the high voltage transformer (Ti) of the starter and of the feed lines (K1, K2) or the like. This resulted in the danger that reliability would be reduced.
On the other hand, in a starter called a DC starter which produces a high voltage, i.e., in which the voltage increases relatively slowly, the insulation breakdown phenomenon is more frequent the higher the voltage and the longer the voltage application time. Here the disadvantage of the possibility of forming of an insulation breakdown in an unintended area is even more increased.
A conventional solution for starting a high pressure discharge lamp using the outside trigger method is shown in the Japanese utility model SHO 37-8045. In this method an arrangement is described in which there is a coil in which at the lamp current of a high pressure mercury lamp a magnetic force is produced and in which operation of a starter circuit at a high pressure produced by the magnetic force of an auxiliary electrode is controlled.
Furthermore, in the approach described in the Japanese patent disclosure document HEI 5-54983 an arrangement is described in which in a lamp such as a high pressure mercury lamp there are auxiliary electrodes (outside electrodes) with a distance of a few millimeters between each other.
However, in each of these conventional approaches for a light source device in which, even for the above described hot restart, the disadvantage of formation of an insulation breakdown in an unintended area and the problem of restarting cannot overcome.
The primary object of the present invention is to eliminate the above discussed disadvantages in the prior art, which are:
that an increase in the size and weight of the starter cannot be avoided in order to prevent heat loss in the windings;
that a restart is difficult when the period after being turned off is short and the lamp is still hot;
that noise is emitted;
that as a result of capacitive coupling between the feed lines and the conductor in the vicinity of the starter a greater amount of energy than necessary must be applied to the feed lines, and
that the possibility of the formation of a insulation breakdown in an unintended area increases; which causes a reduction in reliability.
These disadvantages are overcome by a light source device which comprises the following:
a discharge lamp (Ld) which contains greater than or equal to 0.15 mg mercury per cubic millimeter of the volume of a discharge lamp (12), in which there is a pair of electrodes (E1, E2) positioned opposite one another to create the main discharge with a distance between each other of less than or equal to 2.5 mm, and in which in addition to the above described electrodes for the main discharge there is provided an auxiliary discharge electrode (Et) positioned such that it does not come into contact with the discharge space (12) for the main discharge;
a feed circuit (Bx) for supplying the discharge current to the above described electrodes (E1, E2) for the main discharge; and
a starter (Ue) which produces a high voltage between one of the two electrodes (E1, E2) for providing the main discharge and the above the auxiliary discharge electrode (Et),
so that the light source device has a starter (Ue) which produces a voltage twice to five times as high as a voltage which is necessary for starting the above described main discharge in the lamp when at a room temperature state.
Further, the object of the invention is achieved by a high voltage generating part (Ub) which contains at least one high voltage transformer (Te) in a starter circuit separated from the feed circuit part (By).
Additionally, the object of the invention is achieved by a lamp (Ld) and at least the high voltage transformer (Te) being made as a one-part unit.
In the outside trigger method, the starting property cannot be improved by simply increasing only one of the applied voltages, i.e., the high voltage applied between the first electrode (E1) or the second electrode (E2) and the auxiliary electrode (Et) and the no-load voltage in order to start the main discharge.
It is necessary to apply the above described high voltage and no-load voltage in a suitable ratio according to the length of the time period after turn-off, i.e., according to the lamp conditions, for example, lamp temperature at the time of starting. Furthermore, it has become apparent that, depending on the time period after turn-off, one or both of the high voltage or no-load voltage applied becomes very high, even when a suitable ratio is maintained, and that there is again the above described danger of formation of an insulation breakdown in an unintended area when restart is attempted.
Therefore, the lower boundary of the dielectric strength which can be imparted to a light source device is determined with respect to the compactness and economic efficiency required by the optical device, which results in a minimum time after turn-off before which a restart is possible.